We have generated humanized mice for several P450s that are critical for drug and carcinogen metabolism. These include mice expressing CYP1A1, CYP1A2, CYP2D6, CYP2E1, CYP3A4 and CYP3A7. CYP3A4. CYP3A4 is responsible for the metabolism of approximately 50% of drugs on the market and thus is involved in a large number of clinically relevant drug-drug interactions that occur when two drugs are co-administered that are both CYP3A4 substrates. A CYP3A4 mouse model that expresses human CYP3A4 in the intestine, designated hCYP3A4, was generated and demonstrated to express the CYP3A4 protein and to carry out midazolam 1'-hydroxylation and 4-hydroxylation; these activities were almost completely inhibited by an anti-CYP3A4 monoclonal antibody (Note - This paper was awarded the 2003 Paper of the Year in Drug Metabolism and Disposition). Midazolam pharmacokinetics were altered as compared to wild-type mice when the drug was orally administered; intravenous administration did not reveal a differences in drug elimination thus demonstrating the importance of expression of CYP3A4 in gut on the pharmacokinetics of orally administered drugs. Ketoconazole, a known inhibitor of CYP3A4, increased maximum plasma concentration orally dosed midazolam. These results suggest that the hCYP3A4 mouse would be an appropriate in vivo animal model for the evaluation of human intestine CYP3A4 metabolism of drug candidates and potential food-drug and drug-drug interactions in preclinical drug development.The hCYP3A4 mice displayed a remarkable phenotype that yields clues into the physiological function of this P450. hCYP3A4 female mice were deficient in lactation resulting in low pup survival. (Note - A figure from this paper was awarded the cover picture in Endocrinology). The mammary glands of hCYP3A4 lactating mice were markedly underdeveloped and had low milk content. The impaired lactation phenotype was associated with significantly reduced serum estradiol levels in hCYP3A4 mice. These results suggest that CYP3A4 may play an important role in estradiol homeostasis and suggest caution when treating pregnant or lactating women with CYP3A4 inducers that may increase estradiol metabolism and elimination.Regulation of the CYP3A4 transgene was examined in a second line of mice containing both CYP3A4 and CYP3A7, designated hCYP3A4/CYP3A7. A time course study revealed that CYP3A4 protein and RNA were expressed in the liver of immature male mice and became undetectable when the mice reached maturity at 6 weeks of age. In contrast, CYP3A4 was expressed in both immature and adult females studies were reminiscent of the gender-dependent regulation of P450s in rat. CYP3A4 was also markedly elevated by the constitutive androstane receptor (CAR)-activator phenobarbital in both male and female livers of hCYP3A4/CYP3A7 mice, demonstrating drug induction of the CYP3A4 transgene in this mouse model. Furthermore, continuous infusion of recombinant growth hormone (GH) in hCYP3A4/CYP3A7 male mice, which overrides the low-level pulsatile male plasma GH profile in males, increased hepatic CYP3A4 mRNA and protein to normal female levels. Continuous GH treatment also feminized the expression of endogenous murine Cyp2b and Cyp3a44 genes. These data demonstrated that human CYP3A4 contains all of the cis elements required for it to respond to endogenous hormonal regulators of developmental expression and sexual dimorphism, in particular GH. A potential role for GH in determining the sex-dependent expression of CYP3A4 in human liver was also revealed. These finding are in agreement with studies in humans showing a gender dependent expression of CYP3A4 protein and mRNA in liver. Further, these studies suggest that GH therapy may alter the pharmacokinetic and pharmacodynamic properties of CYP3A4 substrates, leading to enhanced metabolism and disposition of drugs in humans.CYP2E1.